1. Field of the Invention
The present invention relates to a printing apparatus that creates dots to print an image on a printing medium. More specifically the invention pertains to a printing apparatus that enables adjustment of the positions of dot formation in the respective pixels in the main scanning direction.
2. Description of the Related Art
A variety of printers have been used widely to print multi-color, multi-tone images as the output device of the computer and the digital camera. One of such printers is an ink jet printer that causes several color inks to be ejected from a plurality of nozzles formed on a print head, so as to create dots and record an image. In order to attain the high quality printing in this printer, it is desirable to form dots without any significant positional misalignment.
The ink jet printer generally has a print head with a large number of nozzles for the purpose of the improvement in printing speed. FIG. 4 shows one example of the applicable print head. In the example of FIG. 4, a plurality of nozzle arrays, each including a plurality of nozzles Nz arranged at fixed intervals in the sub-scanning direction, are disposed in the main scanning direction. The print head often has a plurality of nozzle rows with regard to each color ink to allow the closely packed arrangement of nozzles. In the example of FIG. 4, each color ink, for example, yellow (Y) ink, has two nozzle rows, that is, a 0th nozzle row and a 1st nozzle row.
FIG. 27 shows a process of printing with a print head having a plurality of nozzle rows. In the example of FIG. 27, a print head HD having two nozzle rows, that is, a 0th nozzle row and a 1st nozzle row, is shifted in a predetermined direction to carry out printing. Symbols A and B respectively represent the positions of the print head HD in the main scanning direction at preset timings. Rectangles P1 and P2 respectively denote pixels. An ink droplet Ip is ejected from a nozzle included in the 0th nozzle row at the preset timing A, so that one dot is formed in the pixel P1. The print head HD then moves in the predetermined direction. At the preset timing B after elapse of a predetermined time period, a nozzle included in the 1st nozzle row reaches the position that has been occupied by the nozzle of the 0th nozzle row at the preset timing A. An ink droplet Ip is ejected from the nozzle included in the 1st nozzle row at the preset timing B, so that another dot is formed in the pixel P1.
In the printer with the print head HD having the plurality of nozzle rows, the timings of ejecting ink droplets from the respective nozzle rows are varied according to an interval D of the adjoining nozzle rows and a moving speed Vc of the print head HD, so that dots are created in each pixel. Namely dots are formed in each pixel by outputting driving signals, which cause the nozzles Nz to eject ink droplets Ip, at a preset time difference between the respective nozzle rows.
In the prior art printer, however, there may be a positional misalignment of dots in the main scanning direction formed by the plurality of nozzle rows, due to reasons discussed below. The positional misalignment results in the poor picture quality. For example, the interval D between the adjoining nozzle rows may be varied, due to an error in the manufacturing process. In another example, the respective nozzles may have varying ink ejection characteristics, that is, ink ejection speed and direction. In the prior art printer, these variations may cause a positional misalignment of dots in the main scanning direction.
FIG. 28 shows the effects of a positional misalignment of dots in the main scanning direction on the picture quality. The open circles represent the dots formed by the 0th nozzle row shown in FIG. 27, whereas the closed circles represent the dots formed by the 1st nozzle row. The left column of FIG. 28 shows an ideal alignment of dots to be formed. The right column of FIG. 28, on the other hand, shows a positional misalignment of dots in the main scanning direction formed by the respective nozzle rows. The positional misalignment is recognized as undesirable bents of a straight line and thereby deteriorates the printing quality. With the development of the high-resolution, high-quality printers, the deterioration of the picture quality due to the positional misalignment of dots in the main scanning direction is not negligible.
In order to cancel the positional misalignment of dots in the main scanning direction, one applicable method regulates the dot formation timings with regard to each nozzle row. This method provides separate driving waveform generation circuits and delay circuits with regard to the respective nozzle rows for regulating the output timings of driving waveforms and individually regulates the dot formation timings with regard to the respective nozzle arrays, in order to prevent the positional misalignment of dots in the main scanning direction. Unlike the prior art printer that ejects ink at a fixed timing preset for each nozzle row, this arrangement enables the regulation of the dot formation timings according to the ink ejection characteristics of each nozzle row.
This method, however, requires the additional circuits to regulate the dot formation timings with regard to the respective nozzle rows. In order to attain the richer tone expression, the printer often uses inks of different densities for some colors. This increases the number of different color inks and thereby the number of nozzle rows on the print head. The additional circuits provided for the respective nozzle rows thus cause a significant increase in manufacturing cost of the printer.
The printing method that forms dots both in the forward pass and in the backward pass of the main scan (hereinafter referred to as the bi-directional printing) has been proposed recently to improve the printing speed. In the case of the bi-directional printing, there may be a positional misalignment of dots in the main scanning direction formed in the forward pass and formed in the backward pass, due to the backlash of the mechanism carrying out the main scan or other causes. Namely the bi-directional printing has the same problem as that occurring in the structure having a plurality of nozzle rows in the main scanning direction.
The positional misalignment of dots is the problem commonly arising when there are two or more different conditions with regard to the dot formation timing, for example, a difference in position in the main scanning direction between adjoining nozzle rows and a difference in moving direction of the print head in the course of dot formation. This problem is found not only between a plurality of nozzle rows with regard to one color ink but between nozzle rows of different color inks. The problem is not restricted to the print head having the arrangement of nozzles shown in FIG. 4, but arises in any print head having nozzles disposed at different positions in the main scanning direction. The problem is found not only in the ink jet printer but a variety of other printing apparatuses that create dots to print an image.
The object of the present invention is thus to provide a technique of preventing a positional misalignment of dots in the main scanning direction and thereby improving the picture quality of the resulting printed image without causing a significant expansion of the circuit structure for driving a print head in a printing apparatus that prints an image with the print head.
At least part of the above and the other related objects is attained by a printing apparatus with a print head having a dot forming element, which creates a dot in response to a driving signal. The printing apparatus carries out main scan, which moves the print head forward and backward relative to a printing medium in a predetermined direction of the printing medium, and creates different dots, which have different dot forming conditions with regard to an ejection timing of ink into each pixel, so as to print an image on the printing medium. The printing apparatus includes: a driving signal output unit that periodically outputs a series of driving signals to the dot forming element in a specific cycle where a plurality of driving signals are allocated to each pixel; an input unit that inputs print data, which represent a density to be expressed in each pixel; a timing storage unit that stores a specific relation between the periodically output driving signals and a pixel with regard to each of the different dot forming conditions; and a head drive unit that carries out the main scan and controls on-off conditions of the plurality of driving signals to create dots in respective pixels according to the input print data, based on the specific relation stored in the timing storage unit.
In the printing apparatus of the present invention, the relation between the driving signals, which are output in a specific cycle where a plurality of driving signals are allocated to each pixel, and the pixel is specified with regard to each of the different dot forming conditions. Namely, the relation determining which part of the successively output driving signals is to be used for formation of dots in each pixel is specified with regard to each of the different dot forming conditions. This arrangement enables the dot forming timing to be regulated in the unit of a time interval when each driving signal is output. The plurality of driving signals are output to each pixel, and the dot formation timing is finely regulated in each pixel according to the number of the output driving signals. The printing apparatus of the present invention thus effectively prevents the positional misalignment of dots in the main scanning direction formed by the different dot forming conditions, thereby improving the picture quality of the resulting printed image. In this printing apparatus, the driving signal is output to the dot forming element at a fixed timing. This accordingly does not require any additional circuit for regulating the dot formation timing with regard to each dot forming condition.
The dot forming conditions represent any conditions that affect the ejection timing of ink to each pixel. The ink ejection timings should be adequately set according to a variety of conditions, in order to ensure formation of dots at predetermined positions in the main scanning direction. For example, the ink ejection timing should be varied with a variation in speed of the main scan of the print head or in speed of ink ejection. In the case where the print head has a plurality of dot forming elements disposed at different positions in the main scanning direction, it is required to vary the ink ejection timing according to the position of the dot forming element in the main scanning direction. In the case of the bi-directional printing, it is also required to change the ink ejection timing between the forward pass and the backward pass of the main scan. These conditions are all included in the different dot forming conditions. Any other conditions that affect the ink ejection timing are also included in the different dot forming conditions.
The printing apparatus of the present invention may be applicable to the variety of dot forming conditions discussed above. In accordance with one preferable application of the printing apparatus, the print head has a plurality of the dot forming elements, each creating a dot in response to the driving signal, arranged in the main scanning direction. The driving signal output unit outputs a common driving signal to the plurality of dot forming elements. The timing storage unit stores the specific relation with regard to each of the plurality of dot forming elements arranged in the main scanning direction.
The following concretely describes the principle of preventing the positional misalignment of dots in the main scanning direction in the printing apparatus of the present invention having the above structure. FIG. 8 shows the principle of adjusting the positions of dot formation in the main scanning direction. Voltage waveforms S1 through S8 shown in the top of FIG. 8 correspond to the successively output driving signals. A print head 28 in the printing apparatus has two rows of dot forming elements having different positions in the main scanning direction, that is, a 0th row and a 1st row. A signal LAT0 specifies the dot formation timing in each pixel with the dot forming element of the 0th row. Another signal LAT1 specifies the dot formation timing in each pixel with the dot forming element of the 1st row. The bottom of FIG. 8 shows pixels and dots formed therein. Each rectangle defined by the broken line represents a pixel, and each hatched circle represents a dot.
In the example of FIG. 8, with regard to the 0th row, dots are formed in the left pixel with the driving signals S1 through S4 and in the right pixel with the driving signals S5 through S8. With regard to the 1st row, on the other hand, dots are formed in the left pixel with the driving signals S3 through S6 and in the right pixel with the driving signals of and after S7. The middle portion of FIG. 8 shows the movement of the print head 28 in four stages from a preset timing xe2x80x98axe2x80x99 to another preset timing xe2x80x98dxe2x80x99. When the print head 28 is located at the position of the timing xe2x80x98axe2x80x99, dot formation with the dot forming element of the 0th row starts in response to the driving signal S1. When the print head 28 moves to the position of the timing xe2x80x98bxe2x80x99, dot formation with the dot forming element of the 1st row starts in response to the driving signal S3. This position is substantially identical with the position of the dot forming element of the 0th row at the timing xe2x80x98axe2x80x99. The dots formed by the dot forming elements of the 1st row are thus practically aligned in the main scanning direction with the dots formed by the dot forming elements of the 0th row.
In this example, the driving signals S1 through S4 are allocated to each pixel with regard to the 0th row, whereas the driving signals S3 through S7 are allocated to each pixel with regard to the 1st row. Regulating the relationship between the driving waveforms and the pixel enables the relative positions of dots in the main scanning direction formed by the dot forming element of the 0th row and formed by the dot forming element of the 1st row to be adjusted finely. The relationship between the driving waveforms and the pixel is regulated by taking into account the interval in the main scanning direction between the dot forming element of the 0th row and the dot forming element of the 1st row, the moving speed of the print head 28, and the positional misalignment of dots formed by the respective dot forming elements. In the example of FIG. 8, the driving signals are output in the cycle where four driving signals are allocated to each pixel. The position of dot formation is thus adjusted in the unit of one-quarter of the width of each pixel.
The xe2x80x98pixelxe2x80x99 is defined as follows in the specification hereof. In the example of FIG. 8, at most four dots can be formed in each pixel. The term xe2x80x98pixelxe2x80x99 generally has a plurality of meanings. In one case, every position where one dot may be formed is defined as a pixel. In this definition, each rectangle in FIG. 8 corresponds to four pixels. In another case, the pixel is defined based on the print data. The on-off conditions of four dots, which may be formed in each rectangle shown in FIG. 8, are determined unequivocally according to the print data of the rectangle. The term xe2x80x98pixelxe2x80x99 in this specification is defined in the latter meaning. Namely the unit of controlling the on-off conditions of the dots is referred to as the pixel. In the definition of this specification, a plurality of dots may be formed in each pixel. In the actual printing operation, the unit of the print data is referred to as the pixel.
The above example regards the case in which four driving signals are allocated to each pixel. The principle of the present invention is, however, applicable to any structure of the printing apparatus that outputs driving signals in a specific cycle where a plurality of driving signals are allocated to each pixel. The greater number of the driving signals allocated to each pixel enhance the accuracy of the adjustment of the positions of dot formation.
The principle of the present invention is applicable to the printing apparatus that enables expression of multilevel tones in each pixel. In the case where a plurality of driving signals are allocated to each pixel as shown in FIG. 8, multilevel densities, for example, xe2x80x98formation of no dotxe2x80x99, xe2x80x98formation of one dotxe2x80x99, xe2x80x98formation of two dotsxe2x80x99, xe2x80x98formation of three dotsxe2x80x99, and xe2x80x98formation of four dotsxe2x80x99, can be expressed with regard to each pixel by controlling the on-off conditions of the dots corresponding to these driving signals. In this case, the timing storage unit stores the relations to enable the multilevel tone expression. The application of the technique of the present invention to the printing apparatus that enables the multilevel expression ensures the smooth tone expression and attains the high quality printing.
The technique of the present invention is, however, not restricted to the printing apparatus that enables multilevel expression. In the example of FIG. 8, the printing apparatus may print an image in only two tone levels, that is, xe2x80x98formation of no dotxe2x80x99 and xe2x80x98formation of four dotsxe2x80x99. In this case, the timing storage unit stores the relations to enable expression of two tone levels. The timing storage unit of the present invention is thus applicable to both the printing apparatus of the two level expression and the printing apparatus of the multilevel expression.
In accordance with another preferable application of the printing apparatus, the timing storage unit stores the specific relation with regard to each of a forward pass and a backward pass of the main scan, and
the head drive unit drives the print head in both the forward pass and the backward pass of the main scan.
In the case of driving the print head both in the forward pass and in the backward pass of the main scan (hereinafter referred to as the bi-directional printing), the printing apparatus of the above application enables the adjustment of the positions of dot formation. The bi-directional printing advantageously improves the printing speed, but may have a positional misalignment of dots in the main scanning direction, for example, due to the backlash of the head-driving mechanism, which results in the poor picture quality. The printing apparatus of the above arrangement can store the relations between the driving signals and the pixel corresponding to the moving directions of the print head to create dots.
In the case where there are two dot forming elements respectively included in the 0th row and the 1st row, the relation with regard to the forward pass and the relation with regard to the backward pass are stored for each of the dot forming elements. This arrangement effectively prevents the positional misalignment of dots formed by the dot forming element of the 0th row and formed by the dot forming element of the 1st row in each of the forward pass and the backward pass of the main scan. The arrangement also prevents the positional misalignment of dots formed in the forward pass and formed in the backward pass of the main scan. The same effects are exerted in the structure having a greater number of dot forming elements. In another structure having only a single row of the dot forming element, this arrangement also effectively prevents the positional misalignment of dots formed in the forward pass and formed in the backward pass of the main scan. The printing apparatus of the above application thus significantly improves the picture quality in the case of the bi-directional printing.
In the printing apparatus that carries out the bi-directional printing, it is preferable that the head drive unit changes the on-off conditions of the driving signals in the backward pass of the main scan from those in the forward pass.
In the case of the bi-directional printing, the relation between the driving waveforms and the pixel in the backward pass of the main scan is reverse to that in the forward pass. Setting the different on-off conditions of the driving signals in the forward pass and in the backward pass of the main scan enables dots recorded in the forward pass and the backward pass to have a consistent configuration, thereby improving the picture quality of the resulting printed image.
This function is described in detail with FIG. 23. In the example of FIG. 23, four driving waveforms are allocated to each pixel, and three out of the four driving waveforms are used for dot formation. The left portion of FIG. 23 shows the dot formation patterns in the forward pass of the main scan, whereas the right portion of FIG. 23 shows the dot formation patterns in the backward pass. The dot of the highest density, that is, the dot corresponding to the print data PD=3, is created with the driving waveforms W1 through W3 in the forward pass of the main scan and with the driving waveforms W2 through W4 in the backward pass. Changing the dot formation patterns in the backward pass from those in the forward pass enables the dots of an identical configuration to be formed at appropriate positions in the respective pixels both in the forward pass and in the backward pass of the main scan. In the example of FIG. 23, part of the driving waveforms allocated to each pixel are used for dot formation. The same function is attained in the structure where all the driving waveforms allocated to each pixel are used for dot formation.
Irrespective of the execution or non-execution of the bi-directional printing, the head drive unit may attain a first state, in which all the plurality of driving signals allocated to each pixel are on, in the process of dot formation in each pixel.
The head drive unit may also attain a second state, in which at least part of the plurality of driving signals allocated to each pixel is always off, in the process of dot formation in each pixel.
The former arrangement enables the tone expression in a wide range to a specific density expressible by setting on all the driving signals allocated to each pixel. This is not restricted to the structure where all the possible combinations of the on-off conditions of the driving signals are stored.
The latter arrangement corresponds to the structure in which the greater number of driving signals than the number of driving signals corresponding to the densities to be expressed are allocated to each pixel. For example, when n driving signals are required to express preset densities in the respective pixels, (n+1) or a greater number of driving signals are allocated to each pixel. In an allowable range of the output frequency of the driving signals, the greater number of driving signals than required are allocated to each pixel. This arrangement enables the finer adjustment of the positions of dot formation.
In the printing apparatus of the present invention, it is preferable that the plurality of driving signals are of an identical type.
This enables the dot formation timing of each dot forming element to be easily regulated.
In accordance with another preferable application of the printing apparatus of the present invention, the driving signal output unit outputs different types of the driving signals allocated to each pixel, and the head drive unit changes the on-off conditions of the driving signals according to the specific relation stored in the timing storage unit.
When the different types of driving signals are periodically output to execute printing, the densities expressed in the respective pixels may be varied according to the relation between the driving signals and the pixel. This phenomenon is described in detail with FIG. 25. Like the example of FIG. 8, in the example of FIG. 25, three different types of driving signals are periodically output in a specific cycle where four driving signals are allocated to each pixel. The three different types of driving signals include a driving signal S1 for creating a smallest-diametral (small-size) dot, driving signals S2 and S3 for creating an intermediate-diametral (medium-size) dot, and a driving signal S4 for creating a largest-diametral (large-size) dot.
The driving signals S1 through S4 are output to the dot forming elements included in the 0th row to create dots, whereas the driving signals S3 through S6 are output to the dot forming elements included in the 1st row to create dots. In the case of the dot forming element of the 0th row, the first driving signal S1 out of all the allocated driving signals is used to create a small dot in a certain pixel. In the case of the dot forming element of the 1st row, on the other hand, the third driving signal S5 out of all the allocated driving signals is used to create a small dot in a certain pixel.
In the printing apparatus of the above arrangement, the on-off conditions of the driving signals are changed according to the relation between the driving signals and the pixel, that is, corresponding to each of the dot forming elements having different positions in the main scanning direction. In the example of FIG. 25, in the case of the dot forming elements of the 0th row, dots are created according to the print data with the driving signals S1 through S4. In the case of the dot forming elements of the 1st row, on the other hand, dots are created according to the print data with the driving signals S3 through S6. Changing the dot formation patterns corresponding to the respective dot forming elements enables the fine adjustment of the positions of dot formation without deteriorating the tone expression even when the plurality of driving signals are of different types. In the above example, four driving signals including three different types are allocated to each pixel. This is, however, only illustrative, and the above arrangement is applicable to any structure in which an arbitrary number of driving signals including an arbitrary number of different types are allocated to each pixel. The above expression xe2x80x98according to the relationxe2x80x99, does not mean that all the possible relations have different on-off conditions of the driving signals.
The principle of the present invention is applicable to a variety of printing apparatuses that create dots.
In one preferable application, the dot forming element ejects ink to create a dot.
The dot forming elements, which eject ink to create dots, often have a positional misalignment of dots in the main scanning direction, due to the ink ejection characteristics. The application of the present invention effectively prevents the positional misalignment of dots and thus remarkably improves the picture quality of the resulting printed image.
A variety of techniques may be applicable to the dot forming element that ejects ink. For example, the dot forming element may eject ink with a pressure of bubbles occurring in ink under a supply of electricity to a heater located in the ink.
It is, however, especially preferable that the dot forming element ejects ink in response to a deflection occurring on application of a voltage, as the driving signal, to a piezoelectric element, so as to create a dot.
The printing apparatus of the present invention periodically outputs the driving signals in a specific cycle where a plurality of driving signals are allocated to each pixel. The dot forming element is accordingly driven at a relatively high frequency. The dot forming element that ejects ink using a piezoelectric element advantageously has a high driving frequency. The application of the principle of the present invention is thus especially effective in the printing apparatus having the dot forming elements utilizing the piezoelectric elements for ink ejection. The application of the present invention to such a printing apparatus effectively prevents the positional misalignment of dots without lowering the printing speed.
In the present invention, the specific relation stored in the timing storage unit may be set in advance for each printing apparatus.
In one preferable application of the present invention, the printing apparatus further includes: a test pattern printing unit that prints a test pattern, which is used to detect a relative positional misalignment of dots, which are created by the print head, in the main scanning direction; and a timing setting unit that sets the specific relation stored in the timing storage unit, based on the printed test pattern.
The printing apparatus of the above application detects the positional misalignment of dots in the main scanning direction using the printed test pattern, and sets the specific relation between the driving signals and the pixel, based on the result of the detection. The positional misalignment of dots in the main scanning direction is ascribed not only to some errors in the manufacturing process but also to the time-based change of the mechanism and ink or other causes occurring in operation of the printing apparatus. The printing apparatus of the above arrangement corrects the specific relation between the driving signals and the pixel using the test pattern, so as to minimize the positional misalignment of dots occurring in operation of the printing apparatus. This arrangement enables the printing quality to be readily kept at a high level and significantly improves the convenience of the printing apparatus.
The present invention is also directed to a method of carrying out main scan that moves a print head with a dot forming element, which creates a dot in response to a driving signal, forward and backward relative to a printing medium in a predetermined direction of the printing medium, and creating different dots, which have different dot forming conditions with regard to an ejection timing of ink into each pixel, so as to print an image on the printing medium. The method includes the steps of: (a) inputting print data that represent a density to be expressed in each pixel; (b) outputting a timing signal that specifies a relation between a series of driving signals, which are periodically output in a specific cycle where a plurality of driving signals are allocated to each pixel, and a pixel with regard to each of the different dot forming conditions; and (c) carrying out the main scan and controlling on-off conditions of the plurality of driving signals to create dots in respective pixels according to the input print data, in response to the timing signal.
Like the printing apparatus discussed above, the method of the present invention effectively prevents the positional misalignment of dots in the main scanning direction, so as to attain the high quality printing. The variety of modifications and additions explained with regard to the printing apparatus are also applicable to the method of the present invention.
The present invention is further directed to a computer readable recording medium, in which a specific program is recorded in a computer readable manner. The specific program is used to drive a printing apparatus with a print head having a dot forming element, which creates a dot in response to a driving signal. The printing apparatus carries out main scan, which moves the print head forward and backward relative to a printing medium in a predetermined direction of the printing medium, and creates different dots, which have different dot forming conditions with regard to an ejection timing of ink into each pixel, so as to print an image on the printing medium. The specific program includes a program code that causes a computer to specify a relation between a series of driving signals, which are periodically output in a specific cycle where a plurality of driving signals are allocated to each pixel, and a pixel with regard to each of the different dot forming conditions.
The computer executes the specific program recorded in the recording medium, so as to specify the relation for each of the dot forming elements, in order to compensate for the positional misalignment of dots. The relation may be specified at every execution of printing or may alternatively follow data previously stored. The specific program having this function is used to drive the printing apparatus, so as to attain the high quality printing. The specific program may be an independent program to actualize the above function or may alternatively be part of the programs for driving the printing apparatus.
Typical examples of the recording media include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. Another possible application of the present invention is a program supply apparatus that supplies a program attaining the above function to the computer via a communication path. The present invention may also be directed to a program attaining the above function or a variety of signals that are equivalent to such a program.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.